Ethernet is a widely used technique in connection of devices inside a network. A single shared cable can serve as the basis for a complete Ethernet network. However, there are certain limitations put on the Ethernet technology. The main limitation is the distance. As the distance increases, the quality of transmission keeps on degrading. Moreover, since Ethernet works on the CSMA/CD protocol (Carrier Sensing Multiple Access with Collision Detection), the number of devices that can transmit successfully at a given point of time is limited to just one. Hence, these kinds of limitations could also limit the wide-spread usage of the Ethernet.
In this post, we will discuss some more things about Ethernet and how to tackle the problem of this limitation of the Ethernet. Over the years, engineers have developed a number of network devices that alleviate this limitation from Ethernet. Many of these technologies are not only specific to Ethernet but can be applied to other networking technologies as well. If you don’t know the basics of Ethernet, it is recommended that you read the post on Ethernet.
What is a Repeater?
The very basic network devices used for the purpose are the repeaters. The first popular medium for Ethernet was known as “Thicknet”. It was a copper coaxial cable and could span about 500 m. But in most of the cases, not all network devices could be spanned within 500 m. Therefore, a Repeater was used to address this problem. A repeater is an electronic device that amplifies an input signal regardless of its nature. The repeaters operate on the physical layer of the network OSI model. Usually, in a large network, there are multiple Ethernet segments.
A LAN segment is a single section of a network that connects computers. So repeaters are used to connect multiple Ethernet segments. What a repeater would generally do is that it will listen from all the Ethernet segments connected to it and will repeat the signal heard on one segment to every other segment connected to it. Using this technique, the effective network diameter can be increased. Also, the repeaters are very versatile devices and allow connections between various types of media. However, the traffic cannot be filtered to ease congestion. Also, a repeater cannot work across multiple network architecture.
Let us suppose that there is a small group of friends. They are talking about the upcoming exam. So when one person is speaking, the other people are listening. If anyone has to speak, he/she will wait for the person currently speaking to stop. This is similar to CSMA/CD protocol. Thus, this way they can successfully prepare well for the exam by communicating properly. But is the number of people in the group is large, say more than 50, and then restricting the number of people who speak at any given point of time to only one will not be a good idea as it will take a huge amount of time for all the people to complete.
This will lead to the creation of small groups within the group talking with each other. Humans have this tendency that they can easily talk, regardless of the number of people around them. Similarly, Ethernet networks also face congestion if the number of devices connected to the network is large. Since the number of devices connected is large, every time the stations find a medium as vacant, they will start transmitting, hence increasing the chances of collision. This can choke out the successful transmission. Hence, the technique of Segmentation was introduced to tackle this problem. A big Ethernet segment can be broken down into multiple segments thus creating Multiple Collision Domains.
Segmentation could have been a successful technique but the only problem was that since the segments were separate, how they share information with each other. Therefore, a device was required which could connect different segments of the Ethernet. This device is known as a Bridge. A bridge, as the name suggests, connects two or more network segments. Bridging happens at the Data Link Layer and the Physical Layer of the OSI Model. Hence they also help in increasing the network diameter. That’s what repeaters do as well. So what’s the difference? The difference is that a bridge, unlike repeaters, regulates traffic as well.
The bridge echoes what it hears from other stations. It creates a special Ethernet frame that allows it to communicate with other bridges. A bridge inspects incoming traffic and decides whether to forward or discard it. An Ethernet bridge inspects each incoming Ethernet frame including the source and the destination MAC addresses and sometimes the frame size in making individual forward decisions. Bridges can also connect different kinds of network media.
We know that Ethernet has a shared medium and works upon the principle of multiple access. This means, that every station on the network might receive the information even though it is not intended to. How do we reduce this unnecessary traffic? If we are using the bridging technique, the bridges possess the ability to filter information by monitoring the destination address of the incoming packet. Let’s take an example with the picture above.
If Computer A has to communicate a packet of data to Computer C, then the information does not have to reach Segment 2 which in case of absence of the bridge, it will. So, the bridge will filter the packet and drop it from going to segment 2. However if the packet Is meant to be delivered to Computer E, the bridge will identify this by looking this at the destination address of the packet and hence, will allow the packet to enter Segment 2. Hence, this reduces the unwanted traffic. This also allows multiple communications between different devices in different segments without increasing congestion.
However, coming to the cons again, even bridges have some limitations concerning segmenting the traffic. An Ethernet broadcasts is meant to be communicated to every node on the network. Hence, in case of an Ethernet broadcast, the bridge will broadcast the packets to all the segments. If the network is too large, broadcasting the packets to all the segments will lead to congestion, as the network will interpret as if all the devices are on a same long segment. Router comes to rescue here. Routers are small physical devices that join multiple networks together.
A router operates at the Network Layer of the OSI Model. A router can divide a single network into two separate logical networks. Since routers put a logical boundary on the network, the Ethernet broadcasts cannot cross them like they cross bridges. Routers can also easily connect networks with different networking architectures because they work on network independent protocols.
Today, in the modern world advanced technologies have replaced these legacy technologies. These advanced technologies include Switched Ethernet and Full Duplex Ethernet. These have helped in increasing the network diameter as well as the transmission rate of data. There are also some technologies which are used as an alternative to Ethernet like the Token- Ring technique, Asynchronous Transfer Mode (ATM) and Fiber Distributed Data Interface (FDDI).
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